Several different amplifier applications require an amplifier having a large gain-bandwidth product. For example, RF signals on optical fibers may require large gain-bandwidth product amplifiers that are highly linear. Some broadband fiber and RF communications applications may require large gain-bandwidth product amplifiers to provide high spectral efficiency. Software configurable communications systems may require an amplifier having a large gain-bandwidth product and a very wide operating bandwidth, which may span baseband frequencies to microwave frequencies. Baseband to microwave instrumentation may require an amplifier having a large gain-bandwidth product and a very wide operating bandwidth.
Distributed amplifiers (DAs) typically use multiple transconductance elements coupled together to provide an amplifier having a larger gain-bandwidth product than is possible with an amplifier using a single comparable transconductance element. A DA may have an input line of inductive elements or transmission line sections coupled in series along with a parallel output line of inductive elements or transmission line sections coupled in series. The input and the output lines have corresponding taps that are coupled to the multiple transconductance elements, such that an input signal, which is applied to one end of the input line, propagates down the input line. As the input signal propagates down the input line, each successive transconductance element receives and amplifies the input signal to feed a corresponding tap into the output line.
Each successive transconductance element adds to the amplified input signal. As such, the amplified input signal propagates down the output line to provide an output signal at the end of the output line. Ideally, the input line and the output line have identical delays, such that the input signal and the amplified input signal stay in phase with one another so that each transconductance element adds to the amplified input signal in phase. However, practical DAs may have phase velocity variations, distortions, or both along the output line that may degrade the linearity of the DA, the efficiency of the DA, or both.
As a result, the gain of a single stage DA even when made from gallium nitride (GaN) or high electron mobility transistor (HEMT) technology may not be sufficient by itself, and an additional DA stage may need to be cascaded with the low noise stage to obtain the desired gain. However, cascading a first stage with a second stage to realize a cascaded two stage amplifier typically compromises noise and linearity in a cascaded RF analysis. Adverse coupling interaction between the first stage and the second stage along with noise contribution of the second stage may result in significant degradation of overall noise figure in comparison with the first stage only. Also the higher gain and signal through the combined first stage and second stage requires an output stage to have better linearity performance than the overall desired cascaded linearity in order to preserve the dynamic range through the cascaded two stage amplifier.
A matrix distributed amplifier is a compact solution which can provide the gain of multiple stages through multiplicative in addition to additive distributed amplification and may alleviate the adverse inter-stage interaction between the first stage and the second stage of a cascaded DA approach by absorbing or incorporating the parasitic effects in a two dimensional distributed fashion. However, the noise performance is limited by the use of traditional resistive terminations of the multiple transmission lines. Thus, it is desirable to have a low noise matrix distributed power amplifier that achieves higher gain and reduced noise figure without significantly compromising other critical performances such as linearity.